1. Field of the Invention
The present invention relates to a compact zoom lens using a folding type inner zoom lens.
2. Description of the Related Art
As the popularity of electronic devices such as personal digital assistants (PDAs) and mobile information units increases, digital cameras or digital video units are included in the electronic device and accordingly the demand for the miniaturization of the camera module increases. Further, providing a zoom camera in such mobile units is increasing. To satisfy the demand, a zoom lens that is ultra-compact compared to a zoom lens used for a general camera is needed. The zoom lens installed on mobile devices has much more restrictions on size compared to the zoom lens installed on digital cameras. That is, in a digital camera, typically only the minimum thickness needs to be considered whereas in a mobile device the size of height and width in addition to the thickness must be made as compact as possible. In order to make a compact zoom lens by reducing the thickness in a digital camera, a optical system that is of a folding type and simultaneously has an inner zoom lens is widely used.
U.S. Patent Publication No. 2004-0105020 A1 discloses an example of the above optical system which includes first through fourth lens groups to embody a compact zoom lens. In general, to harmonize an inner zoom type and a folding type, the first lens group includes a reflection optical component and the first lens group needs to be fixed during zooming or magnification. In general, the second lens group has a negative refractive power and moves during the zooming or magnification so as to function as a variator. The third lens group has a positive refractive power and moves during zooming or magnification so as to function as a compensator. The fourth lens group performs focusing for a structure moving during the zooming or magnification to additionally function as a compensator. In the above structure, the second and third lens groups are driven by a cam while the fourth lens group uses an additional drive motor.
When an optical system in which the fourth lens group is fixed during zooming or magnification is used, the third lens group mainly performs focusing. In this case, the drive motor is connected to each of the second and third lens groups. The second lens group may perform focusing. In this case, however, since the viewing angle of the overall optical system changes, the second lens group is used as a focusing lens only for a special case, for example, for macro photographing. In this case, a stop is located between the second and third lens groups and in most cases the brightness of a lens is maintained by fixing the aperture stop. That is, during zooming or magnification, the second lens group is moved from an object toward the aperture stop while the third lens group is moved from an image side toward the aperture stop.
Japanese Patent Publication No. 2003-202500, as shown in FIG. 1, discloses a zoom lens optical system having first through fifth lens groups Gr1, Gr2, Gr3, Gr4, and Gr5. In detail, the first lens group has a positive refractive power and a reflection optical component, the second lens group has a negative refractive power, the third lens group has an aperture stop and a positive refractive power, the fourth lens group has a positive refractive power, and the fifth lens group has a positive refractive power. In the optical system configured as above, performance is improved by adding a lens in front or rear of an aperture stop ST to correct spherical aberration. That is, the aperture stop functions as a lens group.
During the zooming or magnification from a wide to a telephoto position, the second lens group moves from the object toward an image side and the fourth lens group moves from the image side toward the object while the first, third, and fifth lens groups are fixed. The second lens group functions as a variator while the fourth lens group functions as a compensator. Also, the fourth lens group moves from the image side to the object and simultaneously performs focusing.
However, although the above-described conventional zoom lenses may contribute to the reduction of the thickness of a camera, they have a disadvantage in that they are not suitable for environments with further constraints on reduced height and width.